Trinity, Hiroshima, and Nagasaki
The origins of the term ground zero began with the Manhattan Project and the bombing of Japan. The Strategic Bombing Survey of the atomic attacks, released in June 1946, used the term liberally, defining it as: "For convenience, the term 'ground zero' will be used to designate the point on the ground directly beneath the point of detonation, or 'air zero.'" William Laurence, an embedded reporter with the Manhattan Project, reported that "Zero" was "the code name given to the spot chosen for the atomic bomb test" in 1945.
The Oxford English Dictionary, citing the use of the term in a 1946 New York Times report on the destroyed city of Hiroshima, defines ground zero as "that part of the ground situated immediately under an exploding bomb, especially an atomic one."
The term was military slang, used at the Trinity site where the weapon tower for the first nuclear weapon was at "point zero", and moved into general use very shortly after the end of World War II. At Hiroshima, the hypocenter of the attack was Shima Hospital.
Read more about this topic: Ground Zero